The distribution of hydrocarbons in Titan's atmosphere: An evolutionary algorithm-based model

We propose a new approach to study the chemical complexity of Titan's atmosphere. We have developed an evolutionary algorithm-based model that simulates the evolution of interacting elements with different valences. This abstract model mimics a C–H–O–N system that might get an insight into the general properties of the chemistry of Titan's atmosphere. Comparison with detailed models like photochemical models is discussed to evaluate limitations and benefits of each approach. Comparison with observations suggests that Titan's atmosphere might self-organize to produce hydrocarbons with distributions that follow a power-law relation. If confirmed, this property makes possible some prediction about the abundance of heavy hydrocarbons in the atmosphere of Titan.     

Key reactions in the photochemistry of hydrocarbons in Neptune's stratosphere

We studied the propagation of uncertainties carried by the reaction rate coefficients in the photochemistry of Neptune's stratosphere. We showed that the uncertainties on the mole fractions of main hydrocarbons are equal to or larger than the estimated uncertainties on abundances gathered from observations. From a global sensitivity analysis study, we determined a list of 26 key reactions and discussed the 7 main key reactions that should be studied in priority to lower the uncertainties in the mole fractions computed from a photochemical model. This methodology is essential to improve the predictivity of photochemical models and, consequently, to better understand the physical and chemical processes that govern the composition of giant planet atmospheres.     

Dissociative recombination of nitrile ions with implications for Titan's upper atmosphere

Nitrile ions are abundant in Titan's upper atmosphere and are expected to be lost mainly via dissociative recombination with free electrons. We review in this paper a series of experimental results on the dissociative recombination reactions of nitrile ions known/expected to be present in Titan's upper atmosphere. The experiments were all performed at the heavy ion storage ring CRYRING in Stockholm, Sweden, and the results presented here include information on rate coefficients at electron temperatures relevant for Titan's upper atmosphere as well as information on the product branching fractions of the reactions. We discuss implications of the results for Titan's atmosphere. As an example the presented results support a statement by Krasnopolsky (2009) that nitriles do not degrade to yield N₂ again in Titan's atmosphere, indicating that condensation and polymerization with precipitation to the surface are their ultimate fate.     

Radiocarbon on Titan

Abstract— We explore the likely production and fate of ¹⁴C in the thick nitrogen atmosphere of Saturn's moon Titan and investigate the constraints that measurements of ¹⁴C might place on Titan's photochemical, atmospheric transport and surface‐atmosphere interaction processes. Titan's atmosphere is thick enough that cosmic‐ray flux limits the production of ¹⁴C: absence of a strong magnetic field and the increased distance from the Sun suggest production rates of ?9 atom/cm²/s, ?4x higher than Earth. The fate and detectability of ¹⁴C depends on the chemical species into which it is incorporated: as methane it would be hopelessly diluted even in only the atmosphere. However, in the more likely case that the ¹⁴C attaches to the haze that rains out onto the surface (as tholin, HCN or acetylene and their polymers), haze in the atmosphere or recently deposited on the surface would be quite radioactive. Such radioactivity may lead to a significant enhancement in the electrical conductivity of the atmosphere which will be measured by the Huygens probe. Measurements with simple detectors on future missions could place useful constraints on the mass deposition rates of photochemical material on the surface and identify locations where surface deposits of such material are “freshest”.     

Carbon isotopic enrichment in Titan's tholins? Implications for Titan's aerosols

Since the discovery of the main composition of Titan's atmosphere, many laboratory experiments have been carried out to reproduce its chemical evolution, particularly the formation of organic haze particles found throughout this atmosphere. Some of these simulations have produced solid products—referred to as Titan's tholins—that are assumed to have properties similar to those of Titan's aerosols. In the present work, we focus on the possible isotopic fractionation of carbon during the processes involved in the formation of Titan's tholins. Initial ¹²C/¹³C isotopic ratios measured on tholins made in the laboratory using cold plasma discharges are presented. Measurements of isotopic enhancement in ¹³C (δ¹³C), both on tholins and on the initial gas mixture (N₂:CH₄ (98:2)) used to produce them do not show any clear deficit or enrichment in ¹³C relative to ¹²C in the lab-made tholins compared to the initial gas mixture. Preliminary data recovered from the Aerosol Collector Pyrolyzer (ACP) experiment of the Huygens probe suggests that Titan's aerosols may also be exempt of carbon isotopic enrichment. This observation creates possibilities for deeper analysis of ACP experiment data.     

Clues on the importance of comets in the origin and evolution of the atmospheres of Titan and Earth

Earth and Titan are two planetary bodies formed far from each other. Nevertheless the chemical composition of their atmospheres exhibits common indications of being produced by the accretion, plus ulterior in-situ processing of cometary materials. This is remarkable because while the Earth formed in the inner part of the disk, presumably from the accretion of rocky planetesimals depleted in oxygen and exhibiting a chemical similitude with enstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- and volatile-rich bodies, called cometesimals. From a cosmochemical and astrobiological perspective, the study of the H, C, N, and O isotopes on Earth and Titan could be the key to decipher the processes occurred in the early stages of formation of both planetary bodies. The main goal of this paper is to quantify the presumable ways of chemical evolution of both planetary bodies, in particular the abundance of CO and N₂ in their early atmospheres. In order to do that the primeval atmospheres and evolution of Titan and Earth have been analyzed from a thermodynamic point of view. The most relevant chemical reactions involving these species and presumably important at their early stages are discussed. Then, we have interpreted the results of this study in light of the results obtained by the Cassini–Huygens mission on these species and their isotopes. Given that H, C, N, and O were preferentially depleted from inner disk materials that formed our planet, the observed similitude of their isotopic fractionation, and subsequent close evolution of Earth's and Titan's atmospheres points towards a cometary origin of Earth atmosphere. Consequently, our scenario also supports the key role of late veneers (comets and water-rich carbonaceous asteroids) enriching the volatile content of the Earth at the time of the late heavy bombardment of terrestrial planets.     

A photochemical model of the martian atmosphere

The factors governing the amounts of CO, O2, and O3 in the martian atmosphere are investigated using a minimally constrained, one-dimensional photochemical model. We find that the incorporation of temperature-dependent CO2 absorption cross sections leads to an enhancement in the water photolysis rate, increasing the abundance of OH radicals to the point where the model CO abundance is smaller than observed. Good agreement between models and observations of CO, O2, O3, and the escape flux of atomic hydrogen can be achieved, using only gas-phase chemistry, by varying the recommended rate constants for the reactions CO + OH and OH + HO2 within their specified uncertainties. Similar revisions have been suggested to resolve discrepancies between models and observations of the terrestrial mesosphere. The oxygen escape flux plays a key role in the oxygen budget on Mars; as inferred from the observed atomic hydrogen escape, it is much larger than recent calculations of the exospheric escape rate for oxygen. Weathering of the surface may account for the imbalance. Quantification of the escape rates of oxygen and hydrogen from Mars is a worthwhile objective for an upcoming martian upper atmospheric mission. We also consider the possibility that HOx radicals may be catalytically destroyed on dust grains suspended in the atmosphere. Good agreement with the observed CO mixing ratio can be achieved via this mechanism, but the resulting ozone column is much higher than the observed quantity. We feel that there is no need at this time to invoke heterogeneous processes to reconcile models and observations.     

Laboratory simulations of Titan's atmosphere: organic gases and aerosols

Titan, the main satellite of Saturn, has been observed by remote sensing for many years, both from interplanetary probes (Pioneer and Voyager's flybys) and from the Earth. Its N2 atmosphere, containing a small fraction of CH4 (approximately 2%), with T approximately 90 K and P approximately 1.5 bar at the ground level, is irradiated by solar UV photons and deeply bombarded by energetic particles, i.e. Saturn mangetospheric electrons and protons, interplanetary electrons and cosmic rays. The resulting energy deposition, which takes place mainly below 1000 km, initiates chemical reactions which yield gaseous hydrocarbons and nitriles and, through polymerisation processes, solid aerosol particles which grow by coagulation and settle down to the ground. At the present time, photochemical models strongly require the results of specific laboratory studies. Chemical rate constants are not well known at low temperatures, charged-particle-induced reactions are difficult to model and laboratory simulations of atmospheric processes are therefore of great interest. Moreover, the synthesis of organic compounds which have not been detected to date provides valuable information for future observations. The origin and chemical composition of aerosols depend on the nature of chemical and energy sources. Their production from gaseous species may be monitored in laboratory chambers and their optical or microphysical properties compared to those deduced from the observations of Titan's atmosphere. The development of simulation chambers of Titan's extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titan's atmosphere by remote sensing and in situ analysis in the near future (Cassini-Huygens mission). It appears necessary, as a preliminary step to test on-board experiments in such chambers, and as a final step, when new space data have been acquired, to use them for more general scientific purposes.     

The "F str 4077" star HD 177645 as a possible barium dwarf star

The abundances of 12 elements in the atmosphere of the "F str 4077" star HD 177645 have been determined from new spectroscopic observations with the CCD camera and the model atmosphere method. The overabundance of nitrogen found for this star indicates its possible relation to barium dwarfs with anomalies in the chemical composition of their atmospheres due to mass transfer from the more evolved companion in a binary system. As an object related to Ba stars and CH subgiants, HD 177645 with an effective temperature T_eff=7150K may also have anomalies in the chemical composition characteristic of diffusion processes in chemically peculiar stars of the upper main sequence, as may be indicated by the overabundance of sulfur in its atmosphere.Translated fromAstrofizika, Vol. 39, No. 2, pp. 201–209, April–June, 1996.     

The neutral atmospheres of comets

In this paper we have endeavored to critically evaluate our present understanding of cometary atmospheres. Following a brief introduction of the significance of the study of cometary atmospheres (Section 1), the relevant photometric and spectroscopic observations are summarized in Section 2.The interaction with the solar radiation, with regard to both the excitation of the observed species as well as the dissociation of stable molecules evaporating from the nucleus, is considered in Sections 3 and 4. The gas phase chemistry likely to take place in the dense inner coma is next considered in Section 5.The exospheric and hydrodynamic models of the expanding cometary atmosphere are considered in detail in Section 6, and both their limitations as well as possible improvements are discussed.The observed chemical composition of the neutral atmosphere and the inferred chemical composition of the volatile component of the nucleus, together with possible variations between different classes of comets is next considered in Section 7, and their possible cosmogonic significance is discussed.In conclusion, some of the important directions in which future research should progress, in order to provide more complete and secure knowledge of cometary atmospheres, are stressed (Section 8).Astrophysics and Space Science Review Paper.     

On the (anticipated) diversity of terrestrial planet atmospheres

On our way toward the characterization of smaller and more temperate planets, missions dedicated to the spectroscopic observation of exoplanets will teach us about the wide diversity of classes of planetary atmospheres, many of them probably having no equivalent in the Solar System. But what kind of atmospheres can we expect? To start answering this question, many theoretical studies have tried to understand and model the various processes controlling the formation and evolution of planetary atmospheres, with some success in the Solar System. Here, we shortly review these processes and we try to give an idea of the various type of atmospheres that these processes can create. As will be made clear, current atmosphere evolution models have many shortcomings yet, and need heavy calibrations. With that in mind, we will thus discuss how observations with a mission similar to EChO would help us unravel the link between a planet’s environment and its atmosphere.     

The distribution of Titan's high-altitude (out to ∼50,000 km) exosphere from energetic neutral atom (ENA) measurements by Cassini/INCA

We report observations of Titan's high-altitude exosphere detected out to about 50,000 km altitude. The observations were made by the Ion Neutral Camera (INCA) on board the Cassini spacecraft. INCA detects energetic neutral atoms (ENA) that are formed when the ambient magnetospheric ions charge exchange with Titan's neutral atmosphere and exosphere. We find that Titan's exospheric H₂ distribution follows closely a full Chamberlain distribution including ballistic, escaping and satellite distributions. As expected, neutral densities are dominated by a satellite distribution above about 10,000 km. The maximum detectable extent of the exosphere (～50,000 km) coincides with the radius of the Hill sphere of gravitational influence from Saturn. While we find no direct indications of a neutral Titan torus with densities greater than about 1000 cm^?3, we observe interesting asymmetries in the distribution that warrants further investigation. Based on these findings we compute the average precipitating ENA flux to be about 5×10⁶ keV/(cm² s), or 8×10^?3 erg/(cm² s), which is directly comparable to that of precipitating energetic ions (Sittler, et al., 2009) and slightly higher than that of solar EUV (Tobiska, 2004). Thus, the energy deposited by precipitating ENAs must also be taken into consideration when studying the energy balance of Titan's thermosphere.     

Spectroscopy of molecular oxygen in the Atmospheres of Venus and Mars

The abundances of molecular oxygen in the atmospheres of Venus and Mars are sensitive to fundamental photochemical processes. A new upper limit is reported for the molecular oxygen mixing ratio (O₂/CO₂ < × 10^?7) in the integrated column above the visible cloud tops of Venus, based on spectroscopic observations carried out in early spring, 1982. During the same observing period, an O₂ column abundance of 8.5 cm-am for the atmosphere of Mars was measured, slightly below the O₂ abundances measured a decade earlier.     

The total and relative contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres in the UV and VUV regions

The main aim of this work is to estimate the total contribution of the processes of     molecular ion photodissociation and     collisional absorption charge exchange to the opacity of DB white dwarf atmospheres, and compare this with the contribution of     and other relevant radiative absorption processes included in standard models.
The method for the calculations of the molecular ion     photodissociation cross-sections is based on the dipole approximation and quantum-mechanical treatment of the internuclear motion, while the quasi-classical method for describing absorption processes in     collisions is based on the quasi-static approximation.
Absorption coefficients are calculated in the region  50 nm ≤λ≤ 850 nm  and compared with the corresponding coefficients of other relevant absorption processes; the calculations of the optical depth of the atmosphere layers considered are performed in the far-UV and VUV regions; the contribution of the relevant absorption processes to the opacity of DB white dwarf atmospheres is examined.
We examined the spectral ranges in which the total     and     absorption processes dominate in particular layers of DB white dwarf atmospheres. In addition, we show that in the region of  λ≲ 70 nm  the process of     atom photoionization is also important, in spite of the fact that the ratio of hydrogen and helium abundances in the DB white dwarf atmosphere considered is  1:10⁵  .     

The origin and evolution of the atmospheres of the terrestrial planets

The chemical nature of the Earth's atmosphere is determined by its interaction with the biosphere, hydrosphere and lithosphere. Detailed balance is maintained over long time periods by a complex series of cyclical processes. The chemical differences between the atmosphere of the Earth, on the one hand, and the atmospheres of Venus and Mars, on the other, can be understood in terms of the greater complexity of the terrestrial interactions. When this has been taken into account, the origin of all three planetary atmospheres can be explained as resulting from degassing. Despite the similarity of the atmospheres of Venus and Mars, it seems necessary to invoke different mechanisms for the low amount of water vapour on each.     

Benzene and aerosol production in Titan and Jupiter's atmospheres: a sensitivity study

Benzene has recently been observed in the atmosphere of Jupiter, Saturn and also Titan. This compound is required as a precursor for larger aromatic species (PAHs) that may be part of aerosol particles. Several photochemical models have tried to reproduce the observed quantities of benzene in the atmospheres of Jupiter (both low- and high-latitudes regions), Saturn and Titan. In this present work, we have conducted a sensitivity study of benzene and PAHs formation, using similar photochemical schemes both for Titan and Jupiter (low-latitudes conditions). Two different photochemical schemes are used, for which the modeled composition fairly agrees with observational constraints, both for Jupiter and Titan. Some disagreements are specific to each atmospheric case, which may point to needed improvements, especially in kinetic data involved in the corresponding chemical cycles. The observed benzene mole fraction in Titan's stratosphere is reproduced by the model, but in the case of Jupiter, low-latitudes benzene abundance is only 3% of the observed column density, which may indicate a possible influence of latitudinal transport, since abundance of benzene is much higher in auroral regions. Though, the photochemical scheme of C₆ compounds at temperature and pressure conditions of planetary atmospheres is still very uncertain. Several variations are therefore done on key reactions in benzene production. These variations show that benzene abundance is mainly sensitive to reactions that may affect the propargyl radical. The effect of aerosol production on hydrocarbons composition is also tested, as well as possible heterogenous recombination of atomic hydrogen in the case of Titan. PAHs are a major pathway for aerosol production in both models. The mass production profiles for aerosols are discussed for both Titan and Jupiter. Total production mass fluxes are roughly three times the one expected by observational constraints in both cases. Such comparative studies are useful to bring more constraints on photochemical models.     

Observations of CH4, C2H6, and C2H2 in the stratosphere of Jupiter

We have performed high-resolution spectral observations at mid-infrared wavelengths of CH4 (8.14 micrometers), C2H6 (12.16 micrometers), and C2H2 (13.45 micrometers) on Jupiter. These emission features probe the stratosphere of the planet and provide information on the carbon-based photochemical processes taking place in that region of the atmosphere. The observations were performed using our cryogenic echelle spectrometer CELESTE, in conjunction with the McMath-Pierce 1.5-m solar telescope between November 1994 and February 1995. We used the methane observations to derive the temperature profile of the jovian atmosphere in the 1-10 mbar region of the stratosphere. This profile was then used in conjunction with height-dependent mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. The resulting mixing ratios are 3.9(+1.9)(-1.3) x 10(-6) for C2H6 (5 mbar pressure level), and 2.3 +/- 0.5 x 10(-8) for C2H2 (8 mbar pressure level), where the quoted uncertainties are derived from model variations in the temperature profile which match the methane observation uncertainties.     

Null Variance Altitudes for the photolysis rate constants of species with barometric distribution: Illustration on Titan upper atmosphere modeling

The precision of the rates of the photolysis processes initiating the complex chemistry of Titan’s upper atmosphere conditions strongly the predictivity of photochemical models. Recent studies in sensitivity analysis of such models point out photolysis rate constants as key parameters. However, they have been treated approximately so far. We deal here directly with uncertainty in the absorption cross-sections to derive the uncertain altitude-dependent photolysis rate constants. We observe that the uncertainty on the photolysis rate constants of the major species, N₂ and CH₄, varies strongly with altitude and rather surprisingly vanishes at specific altitudes. We propose a simple model to interpret these features and we demonstrate that they are transferable to any major absorber distributed barometrically in an atmosphere.     

Semi-empirical solar line blanketing

The line blanketing procedure described in an earlier paper has been tested by a series of blanketed solar model atmosphere calculations incorporating methodological and data variations. The results of those tests show the method to be reasonably insensitive to data uncertainties and to certain assumptions employed. The method was also checked by calculating several blanketed stellar atmospheres for comparison with blanketed atmospheres computed by other investigators. Except at the highest atmospheric levels the comparisons of the model structures were good and the blanketing method appears to be validated.Central solar intensities were computed for two semi-empirical solar models and for a theoretical model using the present blanketing method. The results are compared with the observations of Labs and Neckel (1968) in the region 3300 Å to 6500 Å. It is found the blanketing improves agreement of the model predictions with observations in a substantial way, particularly at short wavelengths. Limb-darkening predictions with blanketing were also made for these models at four wavelengths and compared with observations. The blanketing generally produces an important improvement in the comparisons; however, the amount of blanketing to be included for limb-darkening is uncertain.The temperature distribution of the blanketed theoretical model was compared with semi-empirical solar models and a blanketed model by Athay (1970) including non-LTE effects. Over a large range in optical depth the agreement is reasonably good; at small optical depths (₀ 0.01) large divergences are seen which may be due to the effects of non-LTE and to the neglect of strong lines in the present blanketing method. In addition to the good structural comparisons, the quality of the blanketed theoretical model in terms of comparison with central intensity and limb darkening is nearly as good as the semi-empirical models. We conclude that theoretical model atmospheres of solar type or nearly solar type computed with current constant-flux programs and with blanketing included are of high quality.Possible improvements in the blanketing method of importance in some instances are suggested; these include the use of a picket-fence procedure, the inclusion of strong lines, and the consideration of non-LTE effects. Further applications to the solar UV region and to stellar atmospheres are suggested.Publications of the Goethe Link Observatory, Indiana University, No. 131.